1. Field of the Invention
This invention relates generally to drilling systems for forming or drilling wellbores or boreholes for the production of hydrocarbons from subsurface formations and more particularly to drilling systems utilizing sensors for determining downhole parameters relating to the fluid in the wellbore during drilling of the wellbores. The measured fluid parameters include chemical properties including chemical composition (gas, pH, H.sub.2 S, etc.), physical properties including density, viscosity, clarity, lubricity, color, compressibility, accumulation of cuttings, pressure and temperature profiles or distribution along wellbores. This invention further relates to taking actions based at least in part on the downhole measured fluid parameters, including adjusting the properties of the drilling fluid supplied from the surface, fluid flow rate, hole cleaning, and taking corrective actions when a kick is detected, thereby improving the efficiency and effectiveness of the drilling operations.
2. Description of the Related Art
To recover oil and gas from subsurface formations, wellbores (also referred to as boreholes) are drilled by rotating a drill bit attached at an end of a drill string. The drill string includes a drill pipe or a coiled tubing (referred herein as the "tubing") that has a drill bit at its downhole end and a bottomhole assembly (BHA) above the drill bit. The wellbore is drilled by rotating the drill bit by rotating the tubing and/or by a mud motor disposed in the BHA. A drilling fluid commonly referred to as the "mud") is supplied under pressure from a surface source into the tubing during drilling of the to wellbore. The drilling fluid operates the mud motor (when used) and discharges at the drill bit bottom. The drilling fluid then returns to the surface via the annular space (annulus) between the drill string and the wellbore wall or inside. Fluid returning to the surface carries the rock bits (cuttings) produced by the drill bit as it disintegrates the rock to drill the wellbore.
In overburdened wellbores (when the drilling fluid column pressure is greater than the formation pressure), some of the drilling fluid penetrates into the formation, thereby causing a loss in the drilling fluid and forming an invaded zone around the wellbore. It is desirable to reduce the fluid loss into the formation because it makes it more difficult to measure the properties of the virgin formation, which are required to determine the presence and retrievability of the trapped hydrocarbons. In underbalanced drilling, the fluid column pressure is less than the formation pressure, which causes the formation fluid to enter into the wellbore. This invasion may reduce the effectiveness of the drilling fluid.
A substantial proportion of the current drilling activity involves directional boreholes (deviated and horizontal boreholes) and/or deeper boreholes to recover greater amounts of hydrocarbons from the subsurface formations and also to recover previously unrecoverable hydrocarbons. Drilling of such boreholes require the drilling fluid to have complex physical and chemical characteristics. The drilling fluid is made up of a base such as water or synthetic material and may contain a number of additives depending upon the specific application. A major component in the success the drilling operation is the performance of the drilling fluid, especially for drilling deeper wellbores, horizontal wellbores and wellbores in hostile environments (high temperature and pressure). These environments require the drilling fluid to excel in many performance categories. The drilling operator and the mud engineer determine the type of the drilling fluid most suitable for the particular drilling operations and then utilize various additives to obtain the desired performance characteristics such as viscosity, density, gelation or thixotropic properties, mechanical stability, chemical stability, lubricating characteristics, ability to carry cuttings to the surface during drilling, ability to hold in suspension such cuttings when fluid circulation is stopped, environmental harmony, non-corrosive effect on the drilling components, provision of adequate hydrostatic pressure and cooling and lubricating impact on the drill bit and BHA components.
A stable borehole is generally a result of a chemical and/or mechanical balance of the drilling fluid. With respect to the mechanical stability, the hydrostatic pressure exerted by the drilling fluid in overburdened wells is normally designed to exceed the formation pressures. This is generally controlled by controlling the fluid density at the surface. To determine the fluid density during drilling, the operators take into account prior knowledge, the behavior of rock under stress, and their related deformation characteristics, formation dip, fluid velocity, type of the formation being drilled, etc. However, the actual density of the fluid is not continuously measured downhole, which may be different from the density assumed by the operator. Further, the fluid density downhole is dynamic, i.e., it continuously changes depending upon the actual drilling and borehole conditions, including the downhole temperature and pressure. Thus, it is desirable to determine density of the wellbore fluid downhole during the drilling operations and then to alter the drilling fluid composition at the surface to obtain the desired density and/or to take other corrective actions based on such measurements. The present invention provides drilling apparatus and methods for downhole determination of the fluid density during the drilling of the wellbores.
It is common to determine certain physical properties in the laboratories from fluid samples taken from the returning wellbore fluid. Such properties typically include fluid compressibility, rheology, viscosity, clarity and solid contents. However, these parameters may have different values downhole, particularly near the drill bit than at the surface. For example, the fluid viscosity may be different downhole than the viscosity determined at the surface even after accounting for the effect of downhole pressure and temperature and other factors. Similarly, the compressibility of the drilling fluid may be different downhole than at the surface. If a gas zone is penetrated and the gas enters the drilling fluid, the compressibility of drilling fluid can change significantly. The present invention provides drilling apparatus and methods for determining in-situ the above-noted physical parameters during drilling of the wellbores.
Substantially continuous monitoring of pressure gradient and differential pressure between the drill string inside and the annulus can provide indication of to kicks, accumulation of cuttings and washed zones. Monitoring of the temperature gradient can qualitative measure of the performance of the drilling fluid and the drill bit. The present invention provides distributed sensors along the drill string to determine the pressure and temperature gradient and fluid flow rate at selected locations in the wellbore.
Downhole determination of certain chemical properties of the drilling fluid can provide on-line information about the drilling conditions. For example, presence of methane can indicate that the drilling is being done through a gas bearing formation and thus provide an early indication of a potential kick (kick detection). Oftentimes the presence of gas is detected when the gas is only a few hundred feet below the surface, which sometimes does not allow the operator to react and take preventive actions, such as closing valves or shutting down drilling to prevent a blow out. The present invention provides an apparatus and method for detecting the presence of gas and performs kick detection.
Corrosion of equipment in the wellbore is usually due to the presence of carbon dioxide, hydrogen sulphide (H.sub.2 S) and oxygen. Low pH and salt contaminated wellbore fluids are more corrosive. Prior art does not provide any methods for measuring the pH of drilling fluid or the presence of H.sub.2 S downhole. The returning wellbore fluid is analyzed at the surface to determine the various desired chemical properties of the drilling fluid. The present invention provides method for determining downhole certain chemical properties of the wellbore fluid.
As noted above, an important function of the drilling fluid is to transport cuttings from the wellbore as the drilling progresses. Once the drill bit has created a drill cutting, it should be removed from under the bit. If the cutting remains under the bit it is redrilled into smaller pieces, adversely affecting the rate of penetration, bit life and mud properties. The annular velocity needs to be greater than the slip velocity for cuttings to move uphole. The size, shape and weight of the cuttings determine the viscosity necessary to control the rate of settling through the drilling fluid. Low shear rate viscosity controls the carrying capacity of the drilling fluid. The density of the suspending fluid has an associated buoyancy effect on cuttings. An increase in density usually has an associated favorable affect on the carrying capacity of the drilling fluid. In horizontal wellbores, heavier cuttings can settle on the bottom side of the wellbore if the fluid properties and fluid speed are not adequate. Cuttings can also accumulate in washed-out zones. Prior art drilling tools do not determine density of the fluid downhole and do not provide an indication of whether cuttings are settling or accumulating at any place in the wellbore. The present invention utilizes downhole sensors and devices to determine the density of the fluid downhole and to provide an indication whether excessive cuttings are present at certain locations along the borehole.
In the oil and gas industry, various devices and sensors have been used to determine a variety of downhole parameters during drilling of wellbores. Such tools are generally referred to as the measurement-while-drilling (MWD) tools. The general emphasis of the industry has been to use MWD tools to determine parameters relating to the formations, physical condition of the tool and the borehole. Very few measurements are made relating to the drilling fluid. The majority of the measurements relating to the drilling fluid are made at the surface by analyzing samples collected from the fluid returning to the surface. Corrective actions are taken based on such measurements, which in many cases take a long time and do not represent the actual fluid properties downhole.
The present invention addresses several of the above-noted deficiencies and provides drilling systems for determining downhole various properties of the wellbore fluid during the drilling operations, including temperature and pressures at various locations, fluid density, accumulation of cuttings, viscosity, color, presence of methane and hydrogen sulphide, pH of the fluid, fluid clarity, and fluid flow rate along the wellbore. Parameters from the downhole measurements may be computed by a downhole computer or processor or at the surface. A surface computer or control system displays necessary information for use by the driller and may be programmed to automatically take certain actions, activate alarms if certain unsafe conditions are detected, such as entry into a gas zone, excessive accumulation of cuttings downhole, etc. are detected. The surface computer communicates with the downhole processors via a two-way telemetry system.